1. Field of the Invention
This invention relates to a device for controlling the power of heating elements such as resistance elements, halogen lamps and the like, particularly for household electrical appliances such as cooking hobs, ovens and cookers. The control device is of the type described in the introduction to the accompanying main claim.
2. Description of the Related Art
It is well known that the power of a heating element, particularly for household electrical appliances, can be continuously controlled by switching devices, particularly of the type known as buck converters, which produce a controllable output voltage starting from an a.c. source after rectifying and filtering the relative a.c. voltage, and controlling the on/off time ratio of a solid state switch.
The basic scheme of such a device is shown in FIG. 1, in which a rectifier bridge DB and the capacitive input filter CF provide a d.c. voltage Vin from an a.c. source Vac.
The solid state switch S, shown for ease of representation as a static switch, converts the d.c. voltage Vin at the point 1 into a high frequency pulsating voltage, the mean value of which is less than or equal to Vin. The pulsating voltage is then filtered by the capacitive/inductive filter L0 +C0 to obtain a d.c. voltage across the ends of the heating element, here represented by the resistance element R1.
The main drawback of such a device is the production in the mains network of a pulsating current both at low frequency (by the effect of the input capacitor CF) and at high frequency (switching frequency) by the action of the switch S. This means that costly and bulky passive filters have to be connected into the mains network, which apart from their cost reduce the efficiency of the conversion process.
A particular power converter arrangement is also known which enables the same power level required for the particular application to be supplied, but with a reduced amplitude of the pulsating current at the input to the converter stage. Such an arrangement is widely described in U.S. Pat. Nos. 4,184,197, 4,186,437, 4,257,087, 4,274,133 and 4,654,769. In particular, U.S. Pat. No. 4,274,133 describes a possible modification of this arrangement which provides a step-down transfer function. For completeness, this latter prior art arrangement is reproduced in the accompanying FIG. 2. The inductors L1 and L2 can be either coupled or not coupled together. This known arrangement has the intrinsic advantage of reducing the input EMI in the voltage source and the output EMI in the load, mainly if the inductors L1 and L2 (of FIG. 2) are coupled with a defined coupling factor. However this described known converter must be fed with a pure d.c. voltage, as represented by the battery Vg, i.e. with an undulation-free voltage, this requiring the use of further costly filters if a rectifier bridge is used to provide the d.c. voltage from the mains.